FIG. 7 is a circuit diagram showing a gain controllable high-frequency amplifier circuit descried in later-mentioned Patent Document 1. A field-effect transistor 701 amplifies input signals, which are input to a gate. A gain can be controlled by controlling on and off of switches B1 to Bn and controlling a connection of an attenuation capacitor.
In other words, a gain control is executed by controlling an input power. This input power control employs a capacitor control not to lead noise figure (S/N) deterioration in the amplifier circuit. However, when the capacitor of the input of the amplifier circuit changes, it causes a change in an input impedance of the amplifier circuit and an impedance mismatch with a previous circuit is generated. It has been a problem since this causes a linear deterioration of the amplifier circuit.